LOESS AND THE VICKSBURG CAMPAIGN OF MANEUVER

Landing at Bruinsburg, MS, on 30 Apr 1863, U.S. Grant's army began traversing an area covered with up to 100 feet of Pleistocene loess. The loess, an eolian silt deposit, parallels the east bank of the Mississippi River where it is thickest, thinning rapidly eastward. The loess surface once undulated gently, but over geologic time eroded into deep, steep-sloped ravines. In the 19th century, roads in the loess followed the ridges, as other terrain was impassable. The Union army marched until midnight when it encountered fire from Confederate pickets. After a short firefight, both forces waited for daylight. At dawn, the Federals deployed 23,500 men against 7,500 Confederates from Grand Gulf, MS. The Confederates fought stubbornly, using the complicated loess terrain. They fell back slowly and withdrew through Port Gibson about 6:00 PM. Grant's forces pursued, hoping to seize the bridge over Big Black River. They found it burning, but put out the blaze and repaired the bridge. With the Big Black Bridge in Federal hands, the way to Vicksburg was open! Or was it? Grant sent patrols both north across the river and northeast along the south side of the river. The patrol sent north rode to within less than 10 miles from Vicksburg, and reported the terrain as the same steep sided ridges and hollows as around Port Gibson. The other patrol found open, rolling country where artillery could be effective. Also, they reported that the Big Black was only crossable in a few locations between Port Gibson and the Southern Railroad of Mississippi, miles to the northeast. Grant knew his options: A thrust north to Vicksburg could result in another Battle of Port Gibson at every loessal ridge top. A swing into the open country east of the Big Black River might allow his Army to cut the Southern Railroad of Mississippi that was hauling vital supplies into Vicksburg. Meanwhile, his Army would be protected by the unfordable Big Black River on its left flank. Grant knew the Confederates would come out to fight for the railroad, but the battle would likely be on open ground. One of MG Grant's most momentous military decisions had just been decided by the Geology of Mississippi Loess.